Rotary kiln reduction of limonitic ores

ABSTRACT

Directed to the reduction of nickel-containing lateritic ores in a rotary kiln at high production rates to achieve selective reduction of nickel contained in the ore wherein the ore is preheated to at least about 1,000* F. in a preheating zone in the kiln and is maintained at at least this temperature while it travels through the reduction zone and wherein within the reduction zone the preheated ore is first subjected to a strongly reducing atmosphere and is then subjected to a moderately reducing atmosphere adjacent the discharge end of the kiln.

11 nlted States Patent 1 1 3,6561% Curlook eta]. 1451 Apr. 18, 11972 541ROTARYKILN REDUCTION OF 3,185,464 '5/1965 Meyer et a] ..75/34 xLIMONITIC ()RES 2,478,942 8/1949 Queneau eta]... ..75/82 2,850,3769/1958 Queneau eta]... .75/82 Inventors: Walter Curlook, Neullly, a ce;James 2,998,311 8/196] llliseta]. ..75/82 Alexander Evert Bell, PortColborne, 3,503,735 3/1970 Beggset a]. ..75/82 x I3T10,Canada 2,507,1235/1950 Sproule eta]. t ..75/82 X 2,994,601 8/1961 Greene 7.75/31 [73]Assgnee' ai Nckel 3,388,870 6/1968 Thummeta] 1 ..75/21 x 3,390,9797/1968 Greene ..75/3] X [22] Filed: Mar. 12, 1970 I PrimaryExaminer-Henry W. Tarring, I] [21] 18890 Atr0rneyMauriceL. Pine] [30}Foreign Application Priority Data ABSTRACT May 28, 1969 Canada ..047216Directed to the reduction ofnickel'comaining lateritic Ores in a rotarykiln at high production rates to achieve selective 52 us. 01 ..75/82,75/211,75/21 reduction Ofnickelcomainedinthe Ore whereinthe [51] Int CL23/02 C2213 5/18 heated to at least about 1,000 F. in a preheating zonein the [58] Field ofSearch ..75/21 31 82 kiln and is maintained at leastthis temperature while it travels through the reduction zone and whereinwithin the [56] References Cited reduction zone the preheated ore isfirst subjected to a strongly reducing atmosphere and is then subjectedto 21 UNITED S A ES P E TS moderately reducing atmosphere adjacent thedischarge end of the kiln. 2,473,795 6/1949 Hills eta] ..75/82 3,272,6169/1966 Queneau et a1. ..75/91 X 5Claims,3Drawing Figures MU] EXIT GASES1 FOR E1511 gggtgruns comet REDUCED PRODUCT CALCINE ROTARY KILNREDUCTION OF LIMONITIC ORES The present invention is directed to aprocess for extracting metal values from the group consisting of nickel,cobalt and copper from oxide ores wherein the ores are first reduced ina rotary kiln operated under specially controlled conditions for thepurpose of selectively reducing the metal value contained therein torender the same susceptible to extraction by subsequent operations.

The problem of extracting metal values from lateritic ores hasconfronted the art for many years. Ores of the lateritic type, includingboth limonitic ores and silicate ores, are not susceptible toconcentration of metal values contained therein as is the case withsulfide ores. Consequently, all of the ore which is dug from the groundmust be treated to condition the ore for extractive operations such asleaching, carbonyl treatment, etc., so as to recover the desired metalvalues therefrom.

Various methods have been proposed for treating lateritic ores ofvarious types. For example, the Hills and Dufour U.S. Pat. No. 2,473,795discloses a process for selectively reducing the nickel content of Cubanlateritic ores using a Herreschoff type furnace supplied with anatmosphere having a controlled composition with respect to the ratio ofreducing to oxidizing constituents at each shelf in the furnace.According to the Hills et al. patent, recirculation of spent reducinggases is employed to modify and control atmosphere in the finalreduction stages. Such a procedure is impractical in operating a rotarykiln, and is unnecessarily complicated.

M. H. Caron, in a paper titled Ammonia Leaching of Nickel and CobaltOres in Transactions of the AIME, Vol. 188, 1950, pp. 67 90, discussesthe problem of recovering metal values such as nickel and cobalt fromlateritic ores. Caron suggested that initial selective reduction of theore could conveniently be done in a rotary kiln fired from the dischargeend with incompletely combusted fuel such as coal or city gas. Under theCaron procedure for operating a rotary kiln, the ore would be subjectedsimultaneously to the highest reduction temperature and the richestreducing gas on discharge from the kiln, leading almost inevitably inpractice to either overreduction or underreduction of iron contained inthe ore. Also, because the appropriate reduction temperature should notbe exceeded for fear of rendering the nickel inactive to subsequenttreatment, the effective reduction zone, as determined by thetemperature, exists for only a short length of kiln towards thedischarge end resulting in low throughput capability. It is to beunderstood that the amount of iron which is coreduced in the reductivetreatment of lateritic ores, particularly of the limonitic type, toreduce the metal value content can importantly and in many casesdeleteriously affect subsequent treatment of the reduced ore to recoverthe desired metal value or values; Thus, iron reduced along with nickelwill be dissolved by ammonia leach liquors and will precipitate as aflocculent iron hydroxide having a veryslow settling rate. If thequantity of iron hydroxide is excessive, this creates difficulties infiltering and in losses of nickel and cobalt values which co-precipitatewith the iron hydroxide precipitate. Furthermore, excessive productionof metallic iron indicates excessive and unnecessary consumption ofreducing reagents. Again, if vapometallurgical methods are employed forextracting the nickel, excessive iron in the reduced ore will hindersuch extraction and will render subsequent purification and refiningmore difficult.

Because of the complexities involved in the treatment ofnickel-containing lateritic ores so as to recover the metal valuestherefrom in an economical way, many problems remain.

The present invention is directed to the solution of certain of theseproblems as will be made more apparent by the following description andby reference to the drawing in which FIG. 1 depicts, in outline, arotary kiln especially adapted for the selective reduction of metalvalues contained in lateritic ores;

FIG. 2 depicts the atmosphere profile in the kiln during operation interms of the ratio of reducing constituents, e.g.,

carbon monoxide and hydrogen to oxidizing constituents, e.g., carbondioxide, water vapor, etc., in terms of the equivalent ratio of carbonmonoxide to carbon dioxide in the kiln atmosphere; and

FIG. 3 depicts as Case A the temperature profile in a rotary kilnoperated in a standard manner with firing at the discharge end only, anddepicts as Case B the temperature profile in a rotary kiln operated inaccordance with the invention.

Generally speaking, the present invention is directed to the method forcountercurrently operating a rotary kiln for reduction of lateritic orescontaining at least one metal value from the group consisting of nickel,cobalt and copper wherein the sources of fuel for generating thereducing atmosphere and process heat are separated into a plurality ofpaths to produce in a portion of the reduction zone strongly reducingconditions by injection of rich reducing gases to yield in this portionof the reduction zone an atmosphere having a maximum reducing potential,equivalent to a COzCO ratio of at least about 1.5: I preferably at least2: I and a temperature in the range of above about 1,000 F. and belowabout 1,400 F. by the combustion of a minor portion of the reducinggases or of other fuel in the said portion of the reducing zone andwherein in the portion of the reducing zone between the aforedescribedportion and the discharge end of the kiln a more moderate reductionpotential is maintained such that the equivalent CO:CO ratio therein isnot greater than about 1.5:1, and is not less than about 0.5: I.

The concepts of the invention will now be described in relation to thedrawing wherein, in FIG. 1 thereof, reference character 11 depicts, inoutline, a rotary kiln having an inlet ore feed end and a reduced oredischarge end. The interior of the kiln is operated to provide a preheatzone indicated as 21 and a reduction zone indicated as Z2. Within thereduction zone are maintained sub-zones Z3, in which the stronglyreducing atmosphere is maintained, and Z4, in which the moderatereducing potential is maintained. In operation of the apparatus asdepicted in the drawing, a lateritic ore having a fineness not exceedingabout one-half inch and bearing a moisture content of less than about 5percent is fed to the kiln. As the ore proceeds through the preheat zoneof the furnace, it is preheated by the countercurrent flow of hot gasgenerated within the kiln from substantially atmospheric temperature toa temperature over about 1,000 F. whereby it is completely dehydrated.As the ore is fed into the kiln, it may include additions of powderedcoal up to about 5 percent, by weight, representing a minor portion ofthe total fuel requirement, and of a sulfide such as pyrite in amountsto about 1 percent, by weight. The atmosphere in the preheat zone ismaintained neutral or slightly reducing. As the preheated ore enters thereduction zone, it is subjected to much more strongly reducingconditions within the ore bed in order to accomplish the desiredreduction of the metal values contained therein at a maximum rate ofreduction and high throughput rate. In addition, further heat issupplied to the ore by means of combustion of minor quantities of fuelin the reduction zone or by combustion of a minor portion of thereducing gases in the kiln so as to achieve in the reduction zone an oretemperature of over about 1,000 F. but usually less than about 1,400 F.,e.g., between about l,l0O F. and 1,350 F. Maintenance of the strongreducing potential in the ore bed within the reduction zone isaccomplished by injection of rich reducing gases such as producer gas orwater gas or equivalent or similar synthesis gases containing hydrogenand carbon monoxide as the main reductants although hydrogen and carbonmonoxide themselves can be employed. Simultaneously, minor amounts offuel are combusted in the kiln atmosphere by injection of air.Alternatively, mantle burners supplied with oil, producer gas, etc., asfuel and fired with air may be employed in place of direct combustion ofthe reducing gases inside the kiln. As the ore proceeds from the zone 23to the zone Z4, it is subjected to a slightly lower reducing potentialso as to avoid overreduction of iron. This is accomplished by providinga gas with the desired reducing potential at the discharge end of thekiln.

This can be effected by partial combustion of hydrocarbon fuel or of asynthesis gas or by blending rich synthesis gases with lean products ofcombustion of fuels. The total amount of fuel introduced at thedischarge end of the kiln could be up to 50 percent of the overallrequirement. It is to be seen from the foregoing that the fuel and/orrich reducing gases which provide the atmosphere and heat supply to thekiln are divided into a plurality of streams countercurrent to the oreflow through the kiln to yield the special controls in respect ofatmosphere composition and temperature within the kiln. A furthersubstantial proportion of the total fuel required to meet the thermalinput to the kiln may be combusted along the length of the kiln, withthe greatest proportion of such fuel being combusted at the junction ofthe preheat and reduction zones of the kiln to supply heat for dryingand preheating the ore feed in the preheat zone of the kiln. The fuelmay be producer gas or crude oil and may represent about percent toabout 50 percent of the total thermal input to the kiln. The reducinggas supplied to the rich reduction zone of the kiln together with thereductants in the gases introduced at the discharge end of the kiln mustbe sufficient in quantity to effect the required degree of reduction. Toensure this in practice, an excess over the theoretical quantity must beemployed with the result that a quantity of unused reductant passes oninto the preheat zone for further combustion and use in preheating. Thequantity of reducing gases thus supplied can be decreased if solidreducing agents are added to the feed.

By means of the process as described, lateritic ores of the type towhich the invention is particularly directed and containing, on a drybasis, about 1 to 3 percent nickel, about 0.05 to 0.3 percent cobalt,with the iron oxide, magnesia and silica contents being dependent on thetype of mineralization, is reduced such that essentially all of thenickel and cobalt are reduced, but only a small proportion of the ironis found in the metallic state. For example, with limonitic type of ore,containing 42 percent or iron, less than about 15 percent of the ironcontent will be reduced to the metallic state.

It is to be appreciated that the depth of ore being treated should bekept to a minimum in the preheat zone to facilitate drying andpreheating of the ore. Lifters may be employed in the preheat zone toincrease solid-gas contact. In the reduction zone, the depth of the bedmay be increased to increase retention time in this zone, therebyoptimizing the reduction process, by providing one or morecircumferential dams at appropriate points.

By way of example, lateritic ores of the limonitic type containingbetween 38 percent and 46 percent iron and about 1.5 percent nickel wereselectively reduced in a rotary kiln 50 feet in length and about 3 feetin diameter inside the brick lining in preparation for subsequentammonia leaching. The reduction was effected countercurrently, in onecase in standard fashion whereby all of the fuel and reductants wereintroduced at the discharge end of the kiln, and in another caseaccording to the present invention. The typical temperature profile forboth series of tests is depicted in FIG. 3. When operating in thestandard fashion, extractions exceeding percent of the nickel could onlybe achieved at feed rates below about 600 pounds per hour with a maximumextraction of 84 percent being obtained at a feed rate of about 400pounds per hour. Control of the operation was difficult and extractionswere erratic and usually under 80 percent. When operating according tothe present invention, extractions of over 88 percent were readilyachieved at feed rates over 800 pounds per hour, the operation wassteady and the extractions were normally above percent.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim: 1. In the countercurrent fuel fired rotary klln process forselectively reducing metal values from the group consisting of nickel,cobalt and copper contained in a lateritic ore wherein ore fed to therotary kiln is passed successively through a preheat zone and through areduction zone, and reduced ore is discharged from said kiln, theimprovement for maintaining within the reduction zone successive zonesproceeding toward the discharge end of the kiln wherein the atmosphereis first maintained strongly reducing and then moderately reducing whichcomprises supplying the discharge end of the kiln with heat generated byburning a hydrocarbon fuel to supply up to about 50 percent of the totalthermal input for the process and to generate an atmosphere having amaximum ratio of reducing to oxidizing gases equivalent to a COzCO ratioof 1.5:1 in said moderately reducing zone adjacent said discharge end,supplying through said kiln wall rich reducing gases to said stronglyreducing zone, while combusting fuel in said strongly reducing zone tomaintain in said zone an atmosphere wherein the ratio of reducing gasesto oxidizing gases is equivalent to a COzCO ratio of at least about1.521, while maintaining throughout said reduction zone a temperature ofabout l,000 F. to about l,400 F.

2. The process according to claim 1 wherein substantially the balance ofthe fuel required for process heat and atmosphere control is suppliedthrough said kiln wall.

3. The process according to claim 1 wherein the temperature in saidreduction zone is maintained within the range of about l,l00 F. andabout l,350 F.

4. The process according to claim 1 wherein the fuel supplied throughsaid kiln wall is a synthesis gas.

5. The process according to claim 1 wherein the ratio of reducing tooxidizing gases in said strongly reducing zone is equivalent to a COzCOratio of at least about 2: l.

2. The process according to claim 1 wherein substantially the balance ofthe fuel required for process heat and atmosphere control is suppliedthrough said kiln wall.
 3. The process according to claim 1 wherein thetemperature in said reduction zone is maintained within the range ofabout 1, 100* F. and about 1,350* F.
 4. The process according to claim 1wherein the fuel supplied through said kiln wall is a synthesis gas. 5.The process according to claim 1 wherein the ratio of reducing tooxidizing gases in said strongly reducing zone is equivalent to a CO:CO2ratio of at least about 2:1.